1. Field of the Invention
The present invention relates to tying tape-like materials having excellent heat stability and mechanical strength which are prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use. Particularly, the present invention relates to packing and tying band or flat tape, and ribbed tape prepared by using aliphatic polyesters having a sufficiently high molecular weight and having the above properties.
2. Discussion of the Background
In recent years, with the progress of the distribution system and the agriculture plastics have increasingly been used for containers, bags, packing materials and the like which are used for distribution and agricultural work. On the other hand, large amounts of plastics waste from such materials have the possibility of polluting rivers, oceans, soil and are becoming a great social problem. To prevent such pollution the development of biodegradable plastics has been desired; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and a raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because polymeric material sufficient enough to obtain practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as industrial parts, automotive members, domestic articles and the like because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
Tying materials, for example, extruded bands, flat tapes, ribbed tapes and the like (herein after referred to as "tape-like materials" generically) as one of packaging materials and agricultural materials required to have corrosion resistance, high strength and the like mainly consist of a high density polyethylene resin and polypropylene resin from among the polyolefins. The band is used for packaging heavy materials and the tying tape is often applied to strings for binding packages, binder tapes for automatic tying machines. Since polyethylene terephthalate, polypropylene terephthalate and the like are used for high strength tape in specified applications, it goes without saying that polymeric polyesters (referring to polymeric polyesters having number-average molecular weights of at least 10,000) generally used for the plastics are limited to polyethylene terephthalete, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Although there are cases of 2,6-naphthalenedicarboxylic acid being used instead of terephthalic acid, there are no reports of trials which resulted in providing the obtained polymers with biodegradability.
Therefore, it is safe to say that there has been no concept of trying to make the molding of tying tape-like materials using biodegradable aliphatic polyesters in which aliphatic dicarboxylic acid was used practical.
One of the reasons why this application concept has not been thought of is felt to be that in spite of the required special molding conditions and physical properties for the above tying tape-like materials, most of the above-mentioned aliphatic polyesters have melting points of 100.degree. C. or lower even if they are crystalline, and have poor heat stability when melted above that. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters show markedly poor values even when they are the same level of number-average molecular weight as the above-mentioned polyethylene terephthalate, so just conceiving that the molded articles having required strength and the like would be obtained was difficult.
Another reason seems to be that studies for improving the physical properties of the aliphatic polyesters by increasing their number-average molecular weights were not make sufficiently advanced because of their poor heat stability.
The object of the present invention is to provide tying tape-like materials prepared by using the above-mentioned aliphatic polyesters as their components which have sufficient high molecular weights for practical use, have excellent mechanical properties represented by heat stability and tensile strength, and which may be decomposed by microorganisms and the like, i.e., are biodegradable as one means of waste disposal so that they may be easily disposed of after the use thereof.